HR: 11:05h
AN: T32A-04    [Abstracts]
TI: Processes of Strain Localization Recorded on the Atlantis Bank Detachment Fault System, Southwest Indian Ridge
AU: * Miranda, E A
EM: emiranda@uwyo.edu
AF: University of Wyoming Dept. of Geology and Geophysics, 1000 E. University Ave. Dept. 3006, Laramie, WY 82071 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, 210 McLean Hall, Woods Hole, MA 02543 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: University of Wyoming Dept. of Geology and Geophysics, 1000 E. University Ave. Dept. 3006, Laramie, WY 82071 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 210 McLean Hall, Woods Hole, MA 02543 United States
AB: Oceanic detachment faults are widely recognized along ultra-slow- to intermediate-spreading mid-ocean ridges, yet uncertainty surrounds their formation and the process of attendant strain localization. We examine gabbroic rocks of the footwall, hanging wall and detachment fault surface of the Atlantis Bank oceanic detachment fault system, a ~300 km² oceanic core complex formed along the ultra-slow-spreading Southwest Indian Ridge between ~13-10 Ma. The exposed fault rocks provide details of the extensional fault system a distance of ~39 km normal to the ridge axis; core from ODP Hole 735B reveals variation of the fault system with structural depth. We use microstructures and thermometry to describe the structural evolution of the footwall during high-temperature ductile to low-temperature brittle deformation. In addition, we present a detailed rheological study of amphibolite-grade mylonites to identify the dominant deformation mechanisms leading to strain localization to constrain the evolution of the detachment fault system over Atlantis Bank. The thickness of the ductile shear zone associated with denudation of the core complex is ~200 m; the zone of brittle deformation < 50 m thick. Fault rock microstructures suggest, and thermometry confirms, that deformation initiated at submagmatic conditions in the ductile regime, and continued to sub-greenschist temperatures through the semi-brittle and brittle regimes as the fault system was denuded with continued slip. We examine gabbro mylonites and use measured grain size, mineral slip systems and calculated strain rates to interpret the processes responsible for strain localization. We use flow law parameters for plagioclase to construct deformation mechanism maps given the grain size, temperature and lattice preferred orientation (LPO) signature of each naturally deformed sample. Mineral compositions determined using the microprobe are used to estimate temperature during deformation. We detected the presence or absence of a LPO in plagioclase and amphibole with electron backscatter diffraction (EBSD). Four samples selected for detailed study vary in plagioclase composition from An _7-56; thermometry demonstrates that the youngest mylonitic textures develop at average temperatures of 661° to 768° C. The transition in creep mechanism of the naturally deformed rocks corresponds to that of the experimental flow law; given this correlation, we estimate that the strain rates associated with detachment faulting range from 10^-12 to as high as 10^-10 s^-1. The highest differential stresses are similar to those estimated for the Goetze criteria for semi-brittle deformation and Byerlee's Law for frictional sliding, indicating that the onset of brittle deformation promotes high temperature fluid flow and alteration. These results demonstrate that (1) the detachment fault system initiated as a distributed ductile shear zone that, with time and increased displacement, localized into a brittle fault surface, (2) strain localization is dominated by plagioclase rheology and is achieved in part by dynamic recrystallization of plagioclase resulting in the transition from dislocation to diffusion creep, and (3) at high temperatures, the detachment fault system operated under very fast geologic strain rates. These fast strain rates are equivalent to the full plate spreading rate being accommodated on a ~200 m wide fault zone.
DE: 8010 Fractures and faults
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Tectonophysics [T]
MN: Fall Meeting 2005